Catch me if you can: is there a runaway-mass black hole in the Orion Nebula Cluster?

TL;DR

This study uses N-body simulations to explore the early dynamical evolution of the Orion Nebula Cluster, suggesting the possible formation of a massive black hole from runaway stellar collisions that could explain observed stellar motions.

Contribution

It introduces a model where early cluster dynamics lead to runaway collisions and black hole formation, aligning with observed stellar velocity dispersions.

Findings

Runaway stellar collisions likely occurred in the ONC's early evolution.

A massive black hole (>100 solar masses) could have formed and remains in the cluster.

The black hole may be detectable via accretion signatures or stellar motion measurements.

Abstract

We investigate the dynamical evolution of the Orion Nebula Cluster (ONC) by means of direct N-body integrations. A large fraction of residual gas was probably expelled when the ONC formed, so we assume that the ONC was much more compact when it formed compared to its current size, in agreement with the embedded cluster radius-mass relation from Marks & Kroupa (2012). Hence, we assume that few-body relaxation played an important role during the initial phase of evolution of the ONC. In particular, three body interactions among OB stars likely led to their ejection from the cluster and, at the same time, to the formation of a massive object via runaway physical stellar collisions. The resulting depletion of the high mass end of the stellar mass function in the cluster is one of the important points where our models fit the observational data. We speculate that the runaway-mass star may have collapsed directly into a massive black hole (Mbh > 100Msun). Such a dark object could explain the large velocity dispersion of the four Trapezium stars observed in the ONC core. We further show that the putative massive black hole is likely to be a member of a binary system with appr. 70 per cent probability. In such a case, it could be detected either due to short periods of enhanced accretion of stellar winds from the secondary star during pericentre passages, or through a measurement of the motion of the secondary whose velocity would exceed 10 km/s along the whole orbit.